Ive camped in Cornwall most of my adult life, the stand-out sites for me are Gwithian Farm just up from Hayle, super site with huge clean facilities and family shower rooms, Its pretty central and within reasonable distances to a lot of the main attractions. You can drop down from there and head to the Towans beaches or hang a right and head towards the lighthouse which is more rocky. Another great site is Polmanter in St Ives for exactly the same reasons.
Great walks from both with Gwithian having the Red River pub across the road in stones throwing distance.